The present invention relates to focal distance adjustable laser modules, and relates more particularly to such a focal distance adjustable laser module which is inexpensive to manufacture and easy to assemble.
Various laser modules are well known, and intensively used in laser pointers, laser range finders, etc. However, regular laser modules are still not satisfactory in function. For example, in U.S. Pat. No. 5,394,430, the focal distance of the laser module is not adjustable. The focal distance of the laser module of U.S. Pat. No. 5,121,188 can be adjusted through a screw joint. However, because the two threaded members tend to displace relative to each other, the focal distance may change after an adjustment is done. In order to eliminate this problem, the screw joint must be fixed by sealing means when the focal distance is adjusted to the desired range. However, when the screw joint is fixed by sealing means, the focal distance can no loner be adjusted. FIGS. 1A and 1B show another structure of laser module according to the prior art. This structure of laser module comprises a screw cap 101, a lens 102, a cylindrical casing 103, and a mounting plate 104. The screw cap 101 holds the lens 102 on the inside, having an outer thread 114 at the rear end threaded into an inner thread 131 at the front end of the cylindrical casing 103. The mounting plate 104 holds a semiconductor chip 141 and a photodetector 142. A cover 145 which holds a glass 146 is covered on the mounting plate 104 over the semiconductor chip 141 and the photodetector 142, and then mounted with the mounting plate 104 inside the cylindrical casing 103. Because the lens 102 is finished through a grinding process by labor, its focal length is difficult to be accurately obtained. Therefore, when the laser module is assembled, the focal distance must be calibrated. However, when the inner thread 131 and the outer thread 114 are meshed together, there is gaps left in the threads, therefore the cylindrical casing 103 and the screw cap 101 tend to displace relative to each other, causing the adjusted focal distance to be changed. In order to eliminate this problem, the connecting area between the screw cap 101 and the cylindrical casing 103 must be sealed with a sealing agent when the focal distance is adjusted. However, when the connecting area between the screw cap 101 and the cylindrical casing 103 is sealed by a sealing agent, the focal distance is fixed, and can no longer be adjusted again.